PSTN is an acronym for Public Switched Telephone Network. It is the traditional telephone system that the world has been using for decades to make phone calls. In order to access the PSTN from your IP Phone (UA), you need to use a "bridge" between VoIP and PSTN.

If you are a Premium Member, this is accomplished seamlessly by dialing a 1 + areacode + phone number for a North American call or 011 + country code + city code + phone number for an International call.